Cooling down is the second half of the heat-up curve

Every plant has a heat-up curve. It is posted, it is run, it is recorded. Hardly anyone asks about the shutdown — although the same physics applies, only with the opposite sign.

The reason is human: heating up stands at the beginning of a campaign, shutting down at the beginning of a standstill. And a standstill is meant to be short. Every hour the furnace needs to cool is an hour in which nobody can work.

Why cooling fast is more expensive than cooling slowly

As it cools, the hot inner surface contracts while the layers behind it are still warm. That creates tensile stress at the surface — and refractory materials withstand compression far better than tension.

You see the result at the inspection: fine networks of cracks across whole areas that were not there before. They are often not deep, but they are entry paths. In the next campaign melt or gas works its way in, and the hairline crack becomes a break-out.

What can actually be controlled in operation

• Keep doors and flaps closed. The biggest mistake is opening up as soon as the temperature allows it. An open furnace cools many times faster on the door side than at the back — the temperature difference within the wall is greater than during operation.
• No forced cooling with fans. Air hitting a hot surface creates exactly the gradient you are trying to avoid.
• Step the burners down rather than switching them off. As long as the furnace is still receiving heat, the rate can be controlled. After switch-off it is determined by the shell.
• Do not speed up the last stage. Below about 200 degrees the temptation is greatest and the damage is greatest, because the difference between surface and core is then relatively at its highest.

The curve belongs in the same document

In practice it pays to write the cooling curve on the same sheet as the heat-up curve — with the same holding points, read backwards. Then there is no second document that nobody can find, and the shift crew has both directions in front of them.

The permissible rate depends on the material: dense, thick masses need more time than a fibre lining, which has practically no mass. Anyone who has both in one furnace goes by the slowest component, not by the average.

What it means for overhaul planning

Cooling time is not a buffer but a work step with a duration. Plan it in and the standstill is realistic. Treat it as idle time and it will be cut short on shutdown day — and that is the decision nobody records and that is later attributed to nobody.

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